1. Field of the Invention
This invention relates to helicopters in which in-plane rotor vibrations create vertical vibrations in the fuselage and more particularly to improved bifilar absorbers for absorbing such vibrations.
2. Description of the Prior Art
U.S. Pat. No. 3,540,809 issued Nov. 17, 1970 to W. F. Paul et al shows bifilar absorbers on a helicopter main rotor which consist essentially of radial support arms fixed to the rotor hub at the ends of which pendulous masses are supported by tuned pins which extend through circular apertures in the arms and in the masses, the apertures being circular and larger in diameter than the pins so that the pins are free to roll over arcuate surfaces of the apertures as the masses respond to vibrations in the rotor arms.
The bifilar system of the Paul et al patent has the disadvantage that the transmissibility of the bifilar absorber is a strong function of force input from the rotor, achieving maximum efficiency at only one force level. Since the force applied to a helicopter rotor is dependent upon flight conditions, this bifilar system of the Paul et al patent is fully efficient at only one flight condition. This characteristic of the bifilar system is due primarily to the amplitude dependency of the response character of the bifilar absorber.